Scale-up synthesis of high-quality solid-state-processed CsCuX (X = Cl, Br, I) perovskite nanocrystal materials toward near-ultraviolet flexible electronic properties

High-quality CsCu2X3 and Cs3Cu2X5 (X = Cl, Br, I) nanocrystals (NCs) exhibit excellent optoelectronic, physical, and chemical properties for detection of UV radiation due to large carrier mobility and lifetime, and heavy atoms. The nanocrystal materials can be prepared via a low-cost and simple solid-state synthesis. However, poor reproducibility and complex synthesis methods of obtaining perovskite NC thin films represent a drawback for the fabrication of the commercial photoelectric device. To address these issues, we develop highly stable CsCu2X3 and Cs3Cu2X5 NC materials using a facile solid-state reaction method for the scale-up production of halogen lead-free perovskites. We suggest a distinctive way to design a series of nanocrystalline perovskites using short-term synthesis and study the mechanism of perovskite formation using thermal solid-state synthesis. These all-inorganic and lead-free CsCu2X3 and Cs3Cu2X5 exhibit large photoluminescence quantum yields (PLQYs) up to 95.2%. Moreover, flexible paper photodetectors based on this series of lead-free perovskites show strong photoselectivity and bending stability at 254 nm, 365 nm, and 405 nm wavelengths. High-quality responses with a responsivity of 1.1 × 10−3 A W−1 and detectivity of 2.71 × 109 jones under UV illumination (10 μW cm−2) at a bias voltage of 5 mV are demonstrated. These results open prospects for designing photodetectors, LEDs, and other photosensitive devices.


Introduction
Organic-inorganic metal halide perovskites and formamidinium lead halide (FAPbX 3 ) perovskites (PVs) have attracted substantial interest, 1-6 owing to their excellent optical properties, including superior photoluminescence quantum yields (PLQYs), 5-7 tunable bandgap, 8 ease of processability and small exciton binding energy. [9][10][11] All-inorganic lead-containing halides CsPbX 3 (ref. 12) have been widely employed as efficient active materials in solar cells, light-emitting diodes and photodetectors. [13][14][15] However, the key issue of these materials is their instability in ambient conditions related to the organic component. On the other hand, the toxicity of lead (Pb) element is another concern because Pb is a hazardous material. As to PV application, electrical characteristics of electronic devices on exible printable substrate 16 may deteriorate due to external deformation and surface roughness. The change in device shape may produce surface tension and even defects, which result in weakening the photoelectric properties of photodetectors. Although all-inorganic low-dimensional metal halides demonstrate relatively high thermal stability, photoelectric properties of such materials still suffer from low responsivity (R) and detectivity (D*) even at ultra-low operation voltage. 17 The development of new environmentally-friendly materials without organic lead halide compounds is critical for future photovoltaic and display technologies [18][19][20] Numerous groups developed alternative lead-free and organic-inorganic halide perovskites in recent years. Cesium (Cs)-based perovskite nanocrystals (NCs) crystallization technique could utilize suitable solvents to satisfy the need for a diverse variety of crystal geometries and to replace organic materials MA/FA and Pb in the perovskite lattice, such as zero-dimensional (0D) Cs 3 Cu 2 X 5 NCs 21 and subsequent crystallization, 22 inverse temperature crystallization strategy. 23,24 Unfortunately, lead-free perovskite NCs suffer from another possible obstacle. These organicinorganic perovskite NCs and photodetectors based on them still suffered from short-term instability under wet conditions, complex process technology, and a small production rate. 25 It is necessary to develop high-performance, low-cost, gram-scale all-inorganic and lead-free metal halide salts, 26 using such methods as a hot one-pot solution synthesis 27 and a solvent-free mechanochemical approach. 28 A facile all-inorganic halide perovskite solid-state reaction method was reported by Huang 29 and Roccanova. 30 The method is useful and promising to synthesize highquality Cs lead-free copper (Cu) inorganic metal halide PVs. Low-dimensional metal halide perovskites: CsCu 2 X 3 and Cs 3 Cu 2 X 5 (X = Cl, Br, I) exhibited ne stability in the atmospheric environment. [31][32][33] Although the synthesis and bulk properties of low-dimensional Cs copper halides have been reported, the studies about a wide range of compositional variations of halogen anions and the material application as photodetector on exible substrates are limited at present. 34,35 This solid-state reaction technique can be considered as environmentally friendly chemistry because it does not involve the utilization of solvents.
In this paper, orthorhombic single-crystalline lead-free copper halide salts based on CsCu 2 X 3 and Cs 3 Cu 2 X 5 are synthesized by reacting stoichiometric amounts of CsX and CuX (X = Cl, Br, I) using solid-state reaction method for the scale-up halogen lead-free perovskites. High-quality properties are demonstrated for PV devices fabricated on a exible substrate.

Results and discussion
All-inorganic lead-free perovskites were synthesized via a thermal solid-state method, as schematically illustrated in Fig. 1a. The details of growth procedure and synthesis procedure are given in the Experimental section. Briey, the raw materials were mixed in a glovebox and then aer temperature optimization calcined at 400°C (except CsCu 2 Br 3 material, which was calcined at 550°C) for 6 h in a tube furnace to obtain ultra-stable NC materials. This step is nalized by the natural cooling process. The NC lms were subsequently fabricated by spin coating the precursor dissolved in N,N-dimethylformamide (DMF), deposited on a paper substrate and annealing at 100°C Fig. 1d. By precisely controlling the stoichiometric ratio of CsX : CuX powders, the high-quality single crystalline CsCu 2 X 3 and Cs 3 Cu 2 X 5 nanocrystals (NCs) can be obtained, as displayed results in Fig. 1-3. A clear colour change (including green, blue, violet and yellow) over the entire visible range could be controlled owing to the variation of optical absorbance and PL emission. These different photoluminescence spectra emitted from various PV NC powders under UV illumination indicate the formation of tunable bandgaps via facile composition engineering.
Starting with the CsCu 2 Cl 3 and Cs 3 Cu 2 Cl 5 NC materials in Fig. 1a, it was demonstrated that the color of the PL emission (l em ) systematically shis from green (CsCu 2 Cl 3 : 530 nm) to deep green (Cs 3 Cu 2 Cl 5 : 495 nm), from violet (CsCu 2 Br 3 : 435 nm) to blue (Cs 3 Cu 2 Br 5 : 460 nm) and from yellow (CsCu 2 I 3 : 560 nm) to light green (Cs 3 Cu 2 I 5 : 445 nm) in Fig. 1b and c, respectively. NCs can be assigned to an orthorhombic system but different space groups, which is consistent with FFT results. 14 To gain further insights into the optical properties of as-prepared PV NCs, the time-resolved PL (TRPL) and PLQYs were applied. PLQYs can be related to the defects in the materials, as one can nd in Fig. 2, which demonstrates the trend in the behavior. Note, that formation of NCs was conrmed (Fig. 3) using highresolution transmission electron microscopy (HRTEM). The PLQYs increase from 3.69% to 95.2% as substituting Cl with Br and I, opposite to the change in exciton average lifetime. The such phenomenon reects that non-radiation losses reduce and lattice defects decrease, especially strongly for Br and I ioncontaining PV materials.
The emission peaks can be further transferred into the color coordinates in Commission Internationale de L'Eclariage (CIE) 1931. The detailed parameters are summarized in Table S1 and Fig. S1. † It should be noted that CIE 1931 cites similar chromaticity coordinates for CsCu 2 X 3 and Cs 3 Cu 2 X 5 (X = Cl, Br, I) with solution-process-based PVs under 325 nm UV excitation at room temperature (RT). 15 Moreover, to study the carrier dynamics of these six PV NC lms on paper the TRPL measurement results (Fig. 2) were analyzed.
The average photogenerated carrier's lifetime proles can be tted using one or two exponential functions 36 The results demonstrate that the average lifetime of these PVs synthesized using the solid-state method can be about 0.1 microsecond scale. For example, the average lifetime in CsCu 2 Cl 3 is about 111.065 ms. However, by substituting Cl with Br and I, the average lifetime decreases to 0.631 ms. Similar behavior is obtained for Cs 3 Cu 2 X 5 NC materials. It should be emphasized that the exciton lifetime of solid-state processbased NC materials is longer compared to the exciton lifetime registered in lead-free PVs synthesized using the simple solution process. 31 To analyze the carrier lifetime decay of each element in these PVs, the X-ray photoelectron spectroscopy (XPS) of full spectra was further performed. The peak position of Cs 3d and I 3d had a greater shi than those of Cl 2p and Br 3d, as shown in Fig. S2-S7, † in respect to the same electron binding energies of Cu + . A similar trend is also observed for Cs 3 Cu 2 X 5 single crystals, indicating the photoelectrical behavior of metal lead-free halide PVs is determined by halide ions. It should be noted that the atomic radius of Br and I atoms is larger compared to the atomic radius of Cl atom, therefore the atoms have more strong inuence on carrier lifetime than Cl atom. In addition, solid-state phase lead-free metal halide PVs: CsCu 2 X 3 and Cs 3 Cu 2 X 5 NC materials exhibit higher stability in the air compared to those materials obtained using the solution phase. 7,29 Furthermore, two Cu peaks of CsCu 2 X 3 at 932.2 eV and 957.1 eV were resolved in the XPS spectra, which can be assigned to Cu 2p 2/3 and Cu 2p 1/2 , respectively. Both Cu peaks  form in [Cu 2 X 3 ] 1− anion sites, the [CuX 3 ] 1− is closer to the Cu + state rather than the Cu 2+ state, 37 as shown in Fig. S2-S4. † However, Cu 2p 2/3 and Cu 2p 1/2 with a high binding energy of 931.4 eV and 958.2 eV related to the Cu 2p 2/3 and Cu 2p 1/2 , and the Cu 2p signals in Cs 3 Cu 2 X 5 NCs should be attributed to the valence of Cu + . 7 Besides, the higher 933.2 and 953.7 eV, observed in Cs 3 Cu 2 X 5 , and the [Cu 2 X 5 ] 3− are closer to the Cu 2+ state rather than the Cu + state. 37 This results in existing two states for the Cu 2p spectra of Cs 3 Cu 2 X 5 ( Fig. S5-S7 †). 14 Meanwhile, each PV NC material exhibits a similar XPS spectrum with the core levels of Cs 3d and Cu 2p, but different core levels of Cl 2p, Br 3d and I 3d. For example, CsCu 2 Cl 3 exhibited a characteristic peak at ∼200 eV (Cl 2p), but no Br 3d and I 3d related peaks are observed. 16 A similar tendency is also observed for Cs 3 Cu 2 X 5 single crystals, 21 which is in good agreement with the results of energy dispersive X-ray spectroscopy (EDS).
The element composition was subsequently conrmed by EDS, as shown in Fig. S8-S13. † All elements (Cs, Cu, Cl, Br, I) were uniformly distributed in the selected area of NC lms. The corresponding atomic ratios are also evaluated, conrming the stoichiometry of the formed perovskite NC lms.
The High-Resolution TEM (HRTEM) images and Fast Fourier Transform (FFT) patterns of CsCu 2 X 3 and Cs 3 Cu 2 X 5 are shown in Fig. 3. Next, the evolution of the morphology CsCu 2 X 3 and Cs 3 Cu 2 X 5 NC materials is recorded as a function of different reaction times. Images of fabricated materials were obtained aer reaction time at high temperatures: 400°C and 550°C (only for CsCu 2 Br 3 ). With gradually increasing the reaction time from 2 h to 6 h, all PV NCs became larger in size. It is worth noting that some circle-and square-shaped nanosheets of the CsCu 2 Cl 3 and Cs 3 Cu 2 Cl 5 formed (Fig. S14a-d †). On the other hand, both CsCu 2 Br 3 NCs and Cs 3 Cu 2 Br 5 NC materials are gradually connecting with each other. These processes allow forming of nanowires or micro-crosses (Fig. S14e-h †). Notably, the morphologies of CsCu 2 I 3 do not change with a reaction time of 4 h ( Fig. S15i and j †), however, nanorods can be formed aer reacting for 6 h (Fig. S15e †). For the Cs 3 Cu 2 I 5 , the morphologies are controllable at whole reaction stages, and the NCs are uniform ( Fig. S14k and l †). The nal morphology aer 6 h solidphase synthesis of PVs is studied using TEM, the results are shown in Fig. S15. † The proposed solid-state phase method (at 400°C and at 550°Conly for CsCu 2 Br 3 material) for the fabrication of NC materials demonstrates higher stability compared to stabilities published in literature 29 with greatly reduced surface recombination rate.
To investigate the crystal phases of as-prepared perovskite NCs, powder X-ray diffraction (PXRD) was performed (Fig. 3). Each PV NC exhibited a unique PXRD pattern with distinctive characteristic peaks, conrming its crystal phase. The PXRD patterns of solid-state synthesis's Cs 3 Cu 2 X 5 are in good agreement with the experiment data of Lian et al. 7 The crystal structure of Cs 3 Cu 2 X 5 species belongs to the space group Pnma, adopting an orthorhombic zero-dimensional structure. Besides, tetrahedral or triangular [Cu 2 X 5 ] 3− anion sites are isolated by cesium (Cs + ) cations. For the case of CsCu 2 Br 3 and Cs 3 Cu 2 Br 5 , the PXRD pattern shows orthorhombic phase with distinguished peaks: (311) and (221) in Fig. 3b, (222) and (123) in  Fig. 3e, respectively. The PXRD spectrum of CsCu 2 X 3 nanosheets does not suffer severe peak broadening, therefore this allows easier distinguishing their phase from Cs 3 Cu 2 X 5 species. For instance, the one-dimensional electronic structure CsCu 2 X 3 group belongs to the orthorhombic space group Cmcm, and the [Cu 2 X 3 ] 3− anion sites are also separated by rows of Cs + cations. 15 In addition, these PVs exhibit a high thermal resistance aer treatment at a high temperature (400°C or 550°C), as well as good environmental stability aer storing 60 days (Table S1 †). The PXRD and XPS spectra recorded for NC PVs aer two years (shown in Fig. 4) demonstrate that most of the materials have good crystallinity and reproducibility still. It should be emphasized, all PV NC powders have been stored in exposed-toair transparent glass reagent bottles. These PVs materials have been obtained on a conductive tape of 0.3 inch discretely with PV thin-lm thickness of about 2 mm. The powders stored for two years show reproducible XRD and XPS results. The fact conrms the high quality of obtained lead-free PVs.
To deeply study all-inorganic lead-free PVs, exible photodetectors based on these materials were fabricated using the paper substrate, as schematically illustrated in Fig. 5a. The copper electrodes are deposited by electron beam evaporation with a distance of 100 mm. The photodetectors exhibit obvious photoresponse at a xed 40°angle aer hundreds of bending under different wavelengths (254, 365, and 405 nm) irradiation, as shown in Fig. 5. Aer returning to a at state, the optical and electrical performance can be recovered as before. The bandgap (E g ) can be subsequently evaluated by ultraviolet-visible (UV-Vis) absorption spectroscopy (Fig. S16 †) and using an empirical formula E g = hc/l, where is the plank constant, c denotes the velocity of light, and l is the cut-off wavelength. The E g can be determined as 5.02 eV, 4.96 eV and 4.49 eV for CsCu 2 Cl 3 , CsCu 2 Br 3 and CsCu 2 I 3 , 2.12 eV, 3.23 eV and 3.30 eV for Cs 3 -Cu 2 Cl 5 , Cs 3 Cu 2 Br 5 and Cs 3 Cu 2 I 5 crystals, respectively. Since the valence band and conduction band are mainly formed due to the contribution of halides (Cl, Br, I) and Cu 2p orbits, respectively, and these halide and copper atoms are distributed around the hexagonal structure, therefore Cs ions have little inuence on the electronic structure of band edge in the middle of the hexagonal structure.
Noteworthily, devices fabricated based on large E g perovskites are not sensitive to 405 nm and even shorter wavelength irradiation. This can be explained as follows. The excited electrons at 254, 365 and 405 nm wavelengths irradiation could not spill over into the photon-emitting state for CsCu 2 I 3 E g of 4.49 eV and Cs 3 Cu 2 Br 5 E g of 3.23 eV.
To further explore the photoelectrical properties of these devices, the output characteristics and on/off current transient photoresponse of the photodetectors were recorded (Fig. 5b-g). All devices exhibit reproducible on/off cycles, indicating good electrical stability. In addition, the output photoresponse current characteristics were found to be strongly dependent upon the wavelength of the incident illumination. The fact indicates a good wavelength selectivity. The on/off ratio of the CsCu 2 X 3 and Cs 3 Cu 2 X 5 photodetectors are subsequently measured at 10 mW cm −2 and bias voltage (V DS ) 5 mV, and summarized in Table 1, as a function of exciting wavelength.
The R and D* decreased aer the bending with a bending angle of 40°and recorded in Table 1 (the bent direction is parallel to the arrow in Fig. 5b), respectively. The bending of the absorbing surface changes the incident angle and area of the light, which reduced the projected area of light on the PV photodetector. Although the on-off ratio and the response speed of the perovskite devices are far from the desired state-of-art photoelectric application, all these results indicated that the photodetectors using the solid-state method may exhibit good near ultraviolet photoresponse, as well as stable and reproducible photo-switching characteristic as it is shown in this work. It should be noted that photovoltaic properties, allowing to capture of solar energy will enable the future fabrication of wearable biosensors, which do not require a battery. The devices fabricated on the exible substrate will enable the exploration of new pathways for delivering approaches to create highly functional skin-integrated biosensors employing photovoltaic techniques that are capable of working on renewable energy sources, reducing the cost in a green energy and technology strategy. These innovations may be quickly implemented by the industry for commercial production to bring  high-speed, safe and effective biomedical devices to market more quickly and at a cost-efficient basis, to play a major role in solving global challenges in an environmentally-friendly and sustainable way.

Materials
The following commercial materials were utilized without further purication unless otherwise indicated. To fabricate a high-quality perovskite thin lm, the perovskite solution was spin-coated on a paper-based substrate with a rotation speed of 1200 rpm for 35 s, followed by annealing at 100°C for 15 min. The DMF will evaporate from PVs thin lms. The Cu electrode with a thickness of 400 nm was deposited using EBV technology and then the devices with sub-micrometer characteristic size were formed.

Material characterization
The morphology of samples is analyzed using SEM (FEI Nano-SEM 50). Chemical identication of perovskite NCs was conducted by XRD (MAXima XRD-7000). The micro-level structure is characterized by TEM (FEI Tecnai G2F20 S-Twin). For the elemental analysis of the as-synthesized perovskite NCs, XPS data are obtained on a Kratos AXIS-ULTRA DLD photoelectron spectrometer. UV/Vis absorption spectra are obtained using a PerkinElmer UV WinLab, spectrophotometer. PLQYs measurements were performed using HAMAMATSU Quantaurus-QY Plus UV-NIR. The uorescence lifetime image and excitation spectra were employed for Edinburgh FLS1000 spectrouorometer at RT. Reection spectra were obtained by a Shimadzu UV2600 with the external integrating sphere at RT.

Device characterization
The direct current characteristics I-V curves of the perovskite photodetector were carried out using an Agilent parameter analyzer B1500a. The laser diode is controlled to produce a square wave light pulse by a programmable power source Agilent E3640A.

Conclusions
In summary, facile scale-up synthesis of Cs copper halide NC materials using the solid-state method developed in this work is reported. Such perovskite NCs not only exhibited a good thermal stability aer storing in an air environment for two years but also high PLQYs reaching up to 90%. Moreover, a long carrier lifetime of ∼116 ms was obtained, showing a good crystal quality of synthesized NC materials. The exible photodetectors based on these materials undoubtedly exhibited a good photoresponse to UV light, including a responsivity of above 1.0 × 10 −4 A W −1 , a detectivity of 1.0 × 10 9 jones under such low photon power density (10 mW cm −2 ) and V DS = 5 mV. Finally, good air stability and photoswitching characteristics achieved for the photodetectors on exible substrates show great potential for future exible electronics, including bioelectronics and solar energy-powered biosensors on the exible substrate.

Author contributions
Zhi Jiang conceived the idea of scale-up perovskite synthesis and assembled photodetector on exible paper. Hezhuang Liu and Jihua Zou veried the analytical methods. Yixuan Huang contributed to the interpretation of the results. Zhaoquan Xu and Denys Pustovyi carried out the experiment and contributed to the presentation of the original research data; Svetlana Vitusevich contributed to the writing and editing of the paper. All authors have read, contributed to the editing and agreed on the present version of the manuscript.

Conflicts of interest
There are no conicts to declare.